(1) Field of the Invention
The present invention is directed to the technical field of using assemblies of semiconductor light emitting diodes (LEDs) to produce a source of white light, and more particularly to methods and systems to produce color temperature adjustable, LED based, white light sources using suitably powered, and temperature monitored, assemblies of LEDs.
(2) Description of Related Art
Human perception of an object's color results from color sensors in the eye responding to electromagnetic radiation reflected off the object. What color an object appears, therefore, depends on a combination of the reflectance spectrum of the object, the spectral distribution of light in the source illuminating the object and the spectral response of the color sensors in the human eye.
The human eye has three types of color sensors, typically called S, M and L-cones, each of which has a fairly wide spectral response but each of which is more attuned to one of red, green or blue light. There is also a fourth type of sensor, the rods, which are extremely sensitive to light and dominate in low illumination and peripheral view conditions but play an insignificant role in the high illumination, foveal vision situations in which color perception occurs. The stimuli from these sensors combine to provide the brain with the signals it perceives as colors. Human color perception is further complicated by the fact that the combination of signals from the sensors have a response that is non-linear with respect to the intensity of the illuminating light. Moreover, the brain attempts to adapt to account for the type of light illuminating an object. For example, the human brain attempts to see a green apple illuminated by daylight as being the same color green both at midday and closer to dawn or dusk, when the daylight is more yellowish and may also be less intense.
Because of these factors, the light illuminating an object is important in determining the perceived color of an object, and particularly in determining if two colors appear to match. Many people will be familiar with the problem of selecting a jacket to match a pair of black pants while indoors, only to go out into bright daylight and discover that the jacket is in fact navy blue and no longer appears the same color as the trousers.
This problem of using the correct source of light has become of great technical significance in the fields of film and television video production, particularly when scenes are filmed indoors and illuminated by artificial lighting.
The human eye has evolved viewing objects illuminated primarily by daylight. Daylight is a combination of direct radiation from the sun, and skylight, which is sunlight scattered by the atmosphere. The sun is a Planckian, or black-body, radiation source. This is a source that emits light across a broad, continuous range of wavelengths with a spectral distribution that follows Planck's law of radiation and depends only on the temperature of the light emitting body. The sun has a Planckian temperature of 5,800 K. When combined with the Rayleigh scattered sunlight from the sky, it produces a white-light that closely approximates a Planckian radiation source having a temperature of 6,500 K. The Commission Internationale de L'Eclairage (CIE), an international body that develops standards on colorimetry and illuminants, defines such light as CIE Standard Illuminant D65.
Other Planckian sources include candle flames which produce a reddish-yellow light having a color temperature of 2,000 K and tungsten incandescent lighting having a color temperature of 2,500-2,800 K.
By combining tungsten black body light emitters with appropriate filters, cinematographers are able to illuminate indoor scenes with light having the properties of daylight at various times of day, i.e. to produce light sources having color temperatures that range from 3,200 K (dusk/dawn) through 3,400 K (1 hour from dusk/dawn), 5,500 K (noon on a clear day), 6,500 K (noon on an overcast day) to 9,000 to 12,000 K (blue sky).
All light sources may be defined as having a “white point”. This is a pair of chromaticity coordinates that may be used to represent the spectrum, or profile, of the light it emits. The white point may, for instance, be obtained by first normalizing the intensity of the light source's profile, then calculating the degree to which each of the three cones of the eye would be stimulated by that profile. These three values, the so-called tri-stimulus values, may then be used to obtain xy values on a CIE defined normalized, two axis chromaticity diagram.
These white light sources produced by filtered tungsten light may closely resemble light from a Planckian source, and the positions on the CIE chromaticity diagram given by the xy values of their white points form a curve that is termed the Planckian locus.
Other artificial light sources such as fluorescent lights emit light more efficiently than tungsten filament lamps. However, they have spectral distributions that differ significantly from black body radiation. Their white points typically do not lie on the Planckian locus, but they may be characterized as having correlated color temperature (CCT). This is the point on the Planckian locus that is joined to their white point by a straight line drawn normal to the Planckian locus.
Florescent lights are available that have correlated color temperatures (CCT) in a range of 2,900 K, termed a warm-white florescent light, to about 4,300 K, termed a cool-white florescent light. Colors that appear to match under, for instance a warm-white florescent light may not match under a D65 conditions, or under a cool-white florescent lamp.
In trying to characterize and account for these non-Planckian light sources, the CIE developed a Color Rendering Index (CRI) that further characterizes light sources. A light source's CRI is a measure of how closely an object's color as seen illuminated by that light source appears to match the color of that object when illuminated by a D65 light source, i.e., daylight at noon on an overcast day. A D65 light source, therefore has a perfect CRI of 100. The CIE has developed standard sets of colored materials that are used in comparing light sources and calculating their CRI.
Semi-conductor light emitting diodes (LEDs) have emerged over the last few decades as even more efficient and adaptable sources of light. LEDs typically have narrow spectral bands. However, white light can be produced from these relatively monochromatic LED's by, for instance, combining a red LED, a green LED and a blue LED. Such sources may be characterized by a combination of their white point and their CRI.
Although a white light made by combining the output of three separate color LEDs may be adjusted to have a white point at any chosen CCT without the need for additional filters, and even for that white point to lie on the Planckian locus, its CRI is likely to be very poor because of the gaps in the light spectrum between the narrow bands of light emitted by the separate colored LEDs.
Another method of producing white-light sources using LEDs is to combine an LED with a phosphor. Because of the broader spectral output of the LED induced phosphorescence, the CRI of such a white light may be considerably improved, but the CCT is now more dependent on the phosphor and may be difficult to tailor to a specific need.
The technical problem is, therefore, how to combine LEDs to produce, preferably without the need for filters, white light having both a predetermined, adjustable CCT that remains stable once selected, and a high CRI while being reasonably efficient, relatively easy to construct and to be affordable.
The relevant prior art includes:
U.S. Pat. No. 8,174,189 issued to Kim et al. on May 8, 2012 entitled “White LED device capable of adjusting correlated color temperature” that describes a white Light Emitting Diode (LED) device that enables the adjustment of a Correlated Color temperature to realize emotional illumination. The white LED device includes a package body for accommodating a plurality of light source units; a first light source unit accommodated in the package body, configured to have one or more first LED chips and a first phosphor, and configured to emit white light having a first Correlated Color Temperature (CCT); a second light source unit accommodated in the package body, configured to have one or more second LED chips and a second phosphor, and configured to emit white light having a second CCT; and a current control unit for varying current, to be supplied to at least one of the first and second LED chips, so as to adjust the first and second CCTs.
U.S. Pat. No. 8,008,850 issued to Su et al. on Aug. 30, 2011 entitled “Color temperature tunable white light emitting device” that describes a color temperature tunable white light emitting device, including a substrate with an ultraviolet light emitting diode, a purple light emitting diode, and a blue light emitting diode provided over the substrate. The UV LED, the purple LED and the blue LED are coated with a phosphor layer. An omnidirectional reflector is disposed over the phosphor layer. A medium layer is disposed between the omni-directional reflector and the phosphor layer. A transparent substrate is disposed over the omnidirectional reflector and an optical diffuser is disposed over the transparent substrate.
US Patent Application 20110037081 by Wu-Cheng Kuo et al. published on Feb. 17, 2011 entitled “White Light-Emitting Diode Packages with Tunable Color Temperature” that describes a white light-emitting diode package with tunable color temperature, including a package substrate with a first light emitting diode (first LED) disposed over a first portion of the substrate and a second light emitting diode (second LED) disposed over a second portion different from the first portion of the substrate. A phosphor layer is coated around the first and second LED, wherein the phosphor layer is formed by blending at least one colored phosphor grain with a transparent optical resin, and the at least one colored phosphor grain in the transparent optical resin is excited by light from the first and second LED to react and emit white light. In one embodiment, the first and second LED are both blue LEDs for emitting blue light of different wavelengths or ultraviolet (UV) LEDs for emitting UV light of different wavelengths.
U.S. Pat. No. 7,972,022 issued to Pohlert et al. on Jul. 5, 2011 “Stand-mounted light panel for natural illumination in film, television or video” that describes a lighting apparatus comprises a light panel having a panel frame, and a plurality of LEDs or other light elements secured to the panel frame. A self-contained battery unit securably attaches to the outside of the panel frame. The light panel may have a dimmer switch, and may also be capable of receiving power from a source other than the self-contained battery unit. The lighting apparatus can be mounted to a camera or a stand through adapters. Diffusion lenses or color gels can be integrated with or detachable from the light panel. The lighting apparatus may conveniently be provided in the form of a kit, with one or more of a light panel, self-contained battery unit, compact stand, connecting cable(s), adapter(s), lenses or color gels, and so on, provided in a single package.
Various implementations are known in the art, but fail to address all of the problems solved by the invention described herein. Various embodiments of this invention are illustrated in the accompanying drawings and will be described in more detail herein below.